Astrophysics > High Energy Astrophysical Phenomena

Title:A Single Progenitor Model for GW150914 and GW170104

Abstract: The merger of stellar-mass black holes (BHs) is not expected to generate
detectable electromagnetic (EM) emission. However, the gravitational wave (GW)
events GW150914 and GW170104, detected by the Laser Interferometer
Gravitational Wave Observatory (LIGO) to be the result of merging, ~60 solar
mass binary black holes (BBHs), each have claimed coincident gamma-ray
emission. Motivated by the intriguing possibility of an EM counterpart to BBH
mergers, we construct a model that can reproduce the observed EM and GW signals
for GW150914- and GW170104-like events, from a single-star progenitor.
Following Loeb (2016), we envision a massive, rapidly rotating star within
which a rotating bar instability fractures the core into two overdensities that
fragment into clumps which merge to form BHs in a tight binary with arbitrary
spin-orbit alignment. Once formed, the BBH inspirals due to gas and
gravitational-wave drag until tidal forces trigger strong feeding of the BHs
with the surrounding stellar-density gas about 10 seconds before merger. The
resulting giga-Eddington accretion peak launches a jet that breaks out of the
progenitor star and drives a powerful outflow that clears the gas from the
orbit of the binary within one second, preserving the vacuum GW waveform in the
LIGO band. The single-progenitor scenario predicts the existence of variability
of the gamma-ray burst, modulated at the ~0.2 second chirping period of the BBH
due to relativistic Doppler boost. The jet breakout should be accompanied by a
low-luminosity supernova. Finally, because the BBHs of the single progenitor
model do not exist at large separations, they will not be detectable in the low
frequency gravitational wave band of the Laser Interferometer Space Antenna
(LISA). Hence, the single-progenitor BBHs will be unambiguously discernible
from BBHs formed through alternate, double-progenitor evolution scenarios.